PROJECT SUMMARY/ABSTRACT Adult tissue stem cells produce highly differentiated cells throughout life, contributing to the tissue maintenance and repair. To balance between stem cell self-renewal and differentiation, a stem cell often divides asymmetrically, generating one stem cell and one differentiating cell. Failure of this process results in either tissue degeneration or tumorigenic overgrowth. The goal of my research is to reveal molecular and cellular mechanisms that regulate asymmetric stem cell division in the context of the microenvironment, or niche. For the next 5 years, I will primarily focus on the mechanism by which the niche signal is spatially restricted, with emphasis on the novel stem cell In my postdoctoral research on the Drosophila testis, I discovered these previously unrecognized cellular protrusions, termed MT-nanotubes, which extend into the niche only from the germline stem cells and not from the surrounding differentiating cells. Based on my previous work, I hypothesize that the MT-nanotube is the cellular mechanism that limits the niche signal specifically to stem cells: I found that the niche ligand as well as its receptor are located on the surface of MT-nanotubes, likely mediating productive niche signaling such that only stem cells experience a sufficient amount of niche-dependent signal transduction. I will characterize the structure, function and universality of MT-nanotubes. These efforts will reveal a new layer of regulation by which niches transmit signals to stem cells in a highly selective manner. Successful completion of this study will provide a new paradigm for understanding the mechanisms of niche-mediated stem cell self-renewal, and for specific structure, MT (microtubule based)-nanotube. understanding/preventing human pathologies that have been unexplained by existing paradigms of stem cell biology.